1. Field of the Invention
The present invention relates to a technology related to a structure for supporting a fixing unit in an image forming apparatus.
2. Description of the Related Art
Typical electrophotographic image forming apparatuses include a photosensitive element, which is generally in the form of a belt or a drum, a developing device, an intermediate transfer member, which is generally in the form of a belt, and a fixing unit. As the photosensitive element is rotated, its surface is electrically charged and undergoes an optical writing process to have a latent image formed on the surface. The latent image is developed into a toner image by the developing device, and the toner image is transferred at a transfer position onto a recording medium, such as paper or an overhead projector (OHP) transparency, fed to the transfer position. The toner image is transferred to the recording medium directly or indirectly by way of the intermediate transfer member. The toner image is then fixed onto the recording medium in the fixing unit.
Such an image forming apparatus typically has a structure frame as a skeleton that increases rigidity against deformation and distortion. The structure frame includes a base, a pair of upright panels arranged on the base to face each other, and a stay or a bracket arranged across the upright panels. The structure frame is generally made of steel plates and detachably supports modular units in the structure frame. Examples of the modular units include a photosensitive element device, a charger device, an optical writing device, a developing device, a transfer device, a fixing device, an intermediate transfer device, and a recording-medium feed device each of which is individually formed into a modular unit. Some of these devices are combined together in some cases into a process cartridge that is also detachably supported by the structure frame.
Such an electrophotographic image forming apparatus typically includes a fixing device that causes an image formed on a recording medium to be fixed onto the recording medium while passing through a fixing nip formed between a fixing member and a pressure member. This fixing device is generally formed into a modular unit for ease of maintenance and collectively attached to and detached from the structure frame.
However, when the fixing nip in the fixing unit and a conveying roller that conveys a recording medium to the transfer position are not exactly parallel to each other, recording-medium conveying directions of the fixing unit and the conveying roller can deviate. This can disadvantageously result in skew of the recording medium or an abnormal image such as a trapezoidal image distortion. Techniques to solve these problems are disclosed in, for example, in Japanese Patent Application Laid-open No. 2004-13167, Japanese Patent Application Laid-open No. 2006-258998, and Japanese Patent Application Laid-open No. 2000-318888.
Because an image forming apparatus is constructed of a large number of components, dimensional tolerances of components can accumulate and cause a recording-medium conveying direction of a fixing unit relative to be deviated from that of a recording-medium feed device. To this end, it is conceivable to increase precision of the components; however, increasing the precision generally requires additional cost, making it less feasible. Furthermore, to ensure exact parallelism between the recording-medium conveying directions, it is necessary to reduce not only dimensional variations but also dimensional errors during assembly. However, it is practically impossible to remove dimensional errors that can occur during assembly completely.
The parallelism can be increased by using an assembly jig. However, because a high precision is required of the assembly jig in this approach, it is highly difficult to design and manufacture an assembly jig that can satisfy such a requirement. Even when such an assembly jig is used during assembly, it is practically impossible to achieve complete parallelism.
To meet growing demands for high-quality images, ensuring highly exact parallelism between recording-medium conveying devices in an image forming apparatus is required.